The UPS is applied in the power supply of the critical equipment. In FIG. 1, it shows a circuit diagram of a conventional online UPS. In which, it includes a power factor correction (PFC) boost converter (including silicon-controlled rectifiers (SCRs) SCR1-SCR2, inductors L1-L2, switches S1-S2, diodes D1-D2 and capacitors C1-C2), an inverter (including switches Si1-Si2, inductor Li and output capacitor Co), a controller (not shown) and a battery set (including a battery having a positive terminal and a negative terminal, SCRs SCR3-SCR4 and a fuse F2), and it further includes an input electrical magnetic interference (EMI) filter having a bypass AC input terminal, a normal AC input terminal and a neutral input terminal, fuses F1 and F3, inductor Lf, SCRs SCR5-SCR8 and an output EMI filter. In one of an AC mode and a DC mode, the PFC boost converter respectively converts one of an AC and a DC to a positive DC bus and a negative DC bus to provide a stable voltage to the inverter of the next stage.
As shown in FIG. 1, a rectifying circuit of the PFC boost converter is usually connected to the front-end of the battery and includes two rectifying switches. Please refer to the two SCRs SCR1-SCR2 of FIG. 1. After an input AC voltage is rectified by the rectifying circuit, it is required to go through a PFC circuit to be converted to a stable voltage for outputting, and then an AC voltage is outputted by the inverter. Due to the considerations of efficiency and voltage endurance, the choice for a conventional PFC circuit is usually a three-level PFC circuit, which includes an upper boost circuit providing a voltage to the positive DC bus capacitor C1 and a lower boost circuit providing a voltage to the negative DC bus capacitor C2.
For example, the three-level PFC circuit is jointly used by the AC mode when the conventional online UPS as shown in FIG. 1 is operating under the DC mode, and please refer to FIGS. 2(a)-2(b) for the specific working processes. FIG. 2(a) shows current flow directions of the positive half-cycle of the DC mode when the conventional online UPS is working under the DC mode. The smaller dashed path in the central portion of FIG. 2(a) shows a current charging the inductors L1 and L2. The larger dashed path in the surrounding portion of FIG. 2(a) shows a continued current of inductor charging the capacitor C1. FIG. 2(b) shows current flow directions when the conventional online UPS is working under the negative half-cycle of the DC mode. The smaller dashed path in the central portion of FIG. 2(b) shows a current charging the inductors L1 and L2. The larger dashed path in the surrounding portion of FIG. 2(b) shows a continued current of inductor charging the capacitor C2. As shown in FIG. 2(c), the lower switch S2 is continuously turned on and the upper switch S1 is engaged in the pulse-width modulation (PWM) chopping when a voltage is provided to the capacitor C1 connected to the positive terminal of DC bus in the positive half-cycle, and the upper switch S1 is continuously turned on and the lower switch S2 is engaged in the PWM chopping when a voltage is provided to the capacitor C2 connected to the negative terminal of DC bus in the negative half-cycle. As shown in FIG. 2(d), the input terminals of the UPS include an I/L and a neutral I/N, and the neutral should go straightly through the whole UPS until reaching the load side and connect to one of the two terminals of the load (O/L and O/N) from the reliability point of view. Due to that the positive and the negative terminals of the battery are connected to the neutral through the inductors L1 and L2 when the above-mentioned conventional online UPS is in a battery mode, thus the voltage on the neutral generates a voltage jump while switching and results in a quite large EMI. For solving the EMI problem, please refer to the two bypass diodes (DP and DN) connected in series and in front of the two inductors (L1 and L2), and the middle point of the two bypass diodes (DP and DN) is coupled to the neutral I/N. In this way, when switches S1 and S2 are under high frequency switching, one of the two inductors is failed, the EMI problem is greatly improved. Thus, the inductors of the PFC circuit alternately work under working frequency such that the utilization rate of the inductors is only 50% when the conventional online UPS operates under the battery mode.
On the other hand, generally, the customer would expect all the power modules use the same battery set in the battery mode when there are a plurality of UPSs of this type work under parallel connection, which will bring many advantages such as wire saving, easy to maintain, having relatively improved total reliability of the system etc. But this kind of conventional configuration of the UPS can not realize multiple parallel-connected UPSs with a jointly used battery set, and a jointly used neutral would result in a current unbalance of the two UPS modes and cause an out of control status after the multiple UPSs, each of which is shown in FIG. 1, are parallel-connected. In the positive half-cycle of the DC mode as shown in FIG. 2(a), after the current flows through the inductor L1 to reach the neutral, the current requires to go through the parallel-connected circuit of the two inductors of L2s to flow back to the battery when the two inductors L2s of two different UPS modes are parallel-connected between the neutral and the negative terminal of the battery. Although currents flow through the two L1s of the two UPS modes can be controlled by the respective switch S1, but in the two inductors of L2s, how the current shall be shared by each inductor L2 is totally determined by the specific parameters of the two inductors L2s, and a current balance between the two inductors L2s can not be controlled by switches. Similarly, in FIG. 2(b), the current sharing of the two inductors L1s through the parallel-connected circuit configured by the neutral is also determined by the parameters of L1s. The parameters of inductors L1 and L2 are related to the manufacturing technologies and the materials, and are hard to achieve basically the same. Furthermore, the magnetic elements such as the inductors are manufactured by mine materials containing copper and iron, and the costs of which are increasingly higher following the development of the economy. In this way, the currents flow through the inductors of each UPS mode are hardly to be independently controlled, which causes the input power of each UPS to be different from each other, result in inductor current oscillations and the current unbalance of switches, and the single battery set parallel-connected to multiple UPSs can not be realized also.
Thus, finding a method to realize the multiple UPSs parallel-connected to a battery set and to reduce the costs, decrease the volume and to diminish the EMI at the same time is always a target desired to be accomplished by the industry.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicant finally conceived a parallel-connected uninterrupted power supply circuit.